The present invention relates to a method for manufacturing a diffraction grating having periodic corrugations through utilization of two-beam interference exposure, and more particularly to a method of making a diffraction grating which has a structure such that the phases of corrugations in two adjacent regions are reverse to each other.
Since such a diffraction grating having periodic corrugations reflects or passes therethrough light of a desired wavelength alone, it is used, in the field of optical communications, as a filter or an internal element of a distributed feedback semiconductor lasers (hereinafter referred to simply as "DFB lasers").
Of such lasers, a DFB laser of the type having a diffraction grating in a light emitting region or in its vicinity emits light of a single mode, and hence is highlighted as a light source for optical communications, and a variety of proposals have been made on this laser. Especially in recent years, it has attracted attention to reverse the phase of corrugations near the center portion of the diffraction grating for further stabilization of the single mode operation.
The oscillation wavelength of such a DFB laser is determined by the period .LAMBDA. of the corrugations of the diffraction grating, and its stable operation depends upon the accuracy of fabrication of the diffraction grating. Accordingly, the fabrication accuracy of the diffraction grating affects the characteristics of the DFB laser.
As described above, it has been difficult to manufacture, by the conventional two-beam interference exposure technique, the diffraction grating of the type in which the phase of the periodic corrugations is reversed.